In radiographic examinations of the cardiac and vascular systems, catheters, i.e. thin plastic tubing, are utilised in several ways. Contrast liquid can be injected via the catheter for making parts of the vascular system visible on exposure to X-rays. Blood tests can be taken for analysis of dissolved gases, pH value, ion concentration etc. Hydrostatic pressure can be measured by a pressure transducer indicator connected to the catheter.
Catheters are not only used for diagnostic purposes, however. A new form of treatment of vascular constriction utilises catheters which are expandable by a heavy hydrostatic pressure being applied in a cavity, these being known as ballon catheters. In addition, pharmaceutical preparations can be injected through catheters for local drug administration.
For the end of the catheter to reach the desired place in the cardiac and vascular systems, a so-called guide wire is used, the latter being in the form of a wire with a dimension allowing its insertion into the catheter. Typical dimensions for catheters are: outside diameter 1-3 mm, inside diameter 0.5-1.5 mm, length 80-150 cm. The diameter of the guide wire may be about 0.5-1.0 mm. While the catheter is made from a polymer material, the guide wire is most usually made up from a metallic material, generally stainless steel.
In measuring pressure with known apparatus, the pressure is transmitted hydraulically in the catheter. This is problematical, due to the transmission properties of catheter/pressure transducer system. Furthermore, microscopic air bubbles can cause considerable deterioration in the transmission. This measurement is thus not reliable.